This invention relates to a sample valve for solute modulated synchronous detection and more particularly to a means for alternately sampling two gas streams and causing the resultant segments of gas to enter a gas chromatographic detector.
It has been known in physical measurements to recover weak signals from overwhelming noise, or to improve the signal-to-noise ratio by modulating the signal. Subsequent narrow band amplification followed by synchronous demodulation can result in a reduction in noise often by several orders of magnitude. It has also been known that the noise reduction is best obtained if only the signal of interest is modulated and not the noise.
This principle of modulation has also been applied to chromatography. According to the so-called in-line solute modulation scheme described, for example, in U.S. Pat. No. 4,019,863 issued Apr. 26, 1977 to A. Jenkins, et al., and U.S. Pat. No. 4,260,884 issued Apr. 7, 1981 to J. E. Lovelock, the sample modulation is accomplished by periodically modifying, or destroying by electron attachment, the component of interest inside a modulator cell (or solute switch) which is similar to an electron capture detector (ECD), but has a much higher density of electrons available to attach to compounds. The modulator cell functions by periodically destroying compounds that respond strongly (the signal), but not destroying weakly responding compounds (the noise). The compounds that attach electrons are subsequently destroyed by the combination with the positive ions in the cell and thus cannot be detected in the detector such as an ECD placed immediately downstream. The scheme, however, is not satisfactory in that the degree and maximum frequency of modulation are limited by the volume of the modulator cell and the flow rate of gas therethrough. In one aspect of the recent studies, it has been shown that the constraints of improved signal-to-noise ratio and response time require that high modulation frequencies be used. Such high frequencies can be obtained only by utilizing small cell volumes and high gas flow rates. In another aspect, however, it is known that the sample must remain inside the cell as long as possible in the solute destruction portion of the modulation cycle. This naturally requires large volumes and/or slow flow rates.
In FIG. 1, there is shown schematically a detector system developed as a result of an attempt to solve the aforementioned problem of the in-line solute modulation scheme. The sample eluting from a gas chromatographic column (not shown) is split in half by a linear effluent splitter 11 into two cells 12 and 13. One cell (balance cell) 12 is passive in that no change occurs to the sample. In the other cell (modulator cell) 13, however, the component of interest in the sample is selectively destroyed, for example, by electron attachment as explained above. The sample is thereby prevented from responding in the ECD 14 which follows the modulator cell 13. Another detector 15 follows the balance cell 12 and the difference between the output of the two identical detectors 14 and 15 is shown as the final signal. Since the compositions of the gas streams leaving the modulator cell 13 and the balance cell 12 are the same except that the component of interest has been inhibited from responding or removed from the stream leaving the modulator cell 13, only this component produces a difference signal. This method based on measurements of difference in the DC mode involves a problem that precise matching of the detector characteristics is indispensible.
The aforementioned required of precise detector balancing can be obviated if only one detector is used, the gas streams from the two cells 12 and 13 of FIG. 1 being alternately directed into one detector. This method is described in U.S. Pat. No. 4,388,411 issued June 14, 1983 to J. E. Lovelock who had already published its basic concept in J. Chromatogr., 112, 29 (1975), stating therein that the full possibilities of the method would probably await the development of a reliable means of switching carrier-gas flow between two or more paths.